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#14
February 20th 07, 02:04 PM
posted to rec.audio.opinion,rec.audio.pro,rec.radio.amateur.homebrew,rec.radio.amateur.antenna,comp.sys.ibm.pc.soundcard.tech
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Good sound card & software ?
On Mon, 19 Feb 2007 10:34:50 -0800, Richard Clark
wrote: On Mon, 19 Feb 2007 14:06:30 GMT, (Bob Masta) wrote: FFT size is 1024 points, so you won't be able to use this for tuning your guitar, if that's what you had in mind. Hi Bob, Why not? 1024 points (bins) has enough resolution to shake out every note on a Hawaiian slide guitar. The only care is selecting the sampling rate and most FFT packages should be able to resolve exceedingly fine. The problem is that the line resolution of an FFT is the sample rate divided by the number of time points. So with 44100 Hz sample rate and 1024 points you get a bit over 43 Hz per line. So the first non-DC spectral line would be 43 Hz, which is about a low F on a bass guitar, and the very next line would be the F an octave above that... you'd miss an entire octave! The standard musical note frequencies (semitones) differ from each other by about 6% (12th root of 2, since there are 12 notes in an "octave"). But you need much better resolution than this for tuning, typically a "cent" or so, namely 1/100 of a semitone or .06%. At 43 Hz that works out to about 0.026 Hz. A 64K-point FFT at 44100 Hz will have a native resolution of 0.67 Hz, which still isn't good enough... but at 64K samples, you are looking at 1.48 seconds of sound. If the pitch changes over that interval, the spectral peak will be smeared even further. If you attempt to beat this game by going to lower sample rates, note that frequency resolution is still proportional to the inverse of the total sampling interval. So if you tried to use the 1K FFT but sample at 44100 / 64 = 689 Hz to get the same (inadequate) resolution as the above 64K FFT, you'd still need 1.48 seconds of sound. Sound cards don't sample that low intrinsically (typically 8K lower limit, 4K on old Sound Blasters), but you might manage using sample rate conversion (either Windows or sound card built-in). I haven't tried this, but I suspect that it would not be a satisfying experience due to the s-l-o-w time response. Best regards, Bob Masta D A Q A R T A Data AcQuisition And Real-Time Analysis www.daqarta.com Scope, Spectrum, Spectrogram, Signal Generator Science with your sound card! |
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